Tissue With Improved Dispersibility

ABSTRACT

Soft tissue sheets, such as bath tissues, are provided with increased dispersibility and fiber opacity efficiency, while maintaining suitable strength, by the addition of acid-treated seed fibers to the fiber furnish.

BACKGROUND OF THE INVENTION

Most consumers want bath tissue that is not only sufficiently strong forcleaning purposes, but they also want the comfort of mind that thetissues will disperse when flushed down the toilet so that they do notclog sewer or septic lines. While commercially-available bath tissues dodisperse, there is room for improvement. Unfortunately, increaseddispersibility usually comes with a decrease in strength, which isundesirable. Therefore there is a need for bath tissues having adequatestrength with increased dispersibility, while at the same timeexhibiting good opacity for perceived hand protection in use.

SUMMARY OF THE INVENTION

It has now been discovered that soft tissues, such as bath tissue, canbe made with improved dispersibility and strength as compared tocurrently available commercial bath tissue products. In addition, thefiber opacity efficiency (hereinafter defined) can also be improved.

Hence in one aspect, the invention resides in a soft tissue having abone dry basis weight from about 15 to about 35 grams per square meter,a geometric mean tensile strength from about 500 to about 1000 grams per3 inches of width and a Dispersibility (hereinafter defined) of about1.5 cycles or less.

In another aspect, the invention resides in a soft tissue having a bonedry basis weight from about 15 to about 35 grams per square meter andfrom about 0.5 to about 5 dry weight percent of an Enhanced FiberAdditive (hereinafter defined), said tissue having a geometric meantensile strength from about 500 to about 1000 grams per 3 inches ofwidth and a Dispersibility of about 1.5 cycles or less.

For purposes herein, a “soft tissue” is sheet of cellulosic papermakingfibers suitable for use as a bath tissue. Such soft tissue sheets arecharacterized by a relatively high bulk and low stiffness (as measuredby the geometric mean slope). More specifically, the soft tissue sheetbulk can be about 3 cubic centimeters or greater per gram of fiber, morespecifically from about 4 to about 20 cubic centimeters per gram offiber (cc/g), and still more specifically from about 5 to about 10 cc/g.The geometric mean slope of the soft tissue sheet can be from about 1 toabout 10 kilograms, more specifically from about 1.5 to about 8kilograms, and still more specifically from about 2 to about 6kilograms.

For purposes herein, an “Enhanced Fiber Additive” is a known seed-basedfiber additive, such as fibers derived from corn or soybeans, which havebeen modified by acid treatment. The acid treatment may optionally befollowed by a mild acid chlorite solution, a peroxide solution, or acombination of both. The resulting Enhanced Fiber Additive is high inhemicellulose, which increases fiber-to-fiber bonding, and is normallyused as a strength agent in high density papers. The production and usesof Enhanced Fiber Additives is disclosed in U.S. Pat. No. 6,902,649 B1entitled “Enhanced Fiber Additive; and Use”, issued Jun. 7, 2005 toSatyavolu et al., which is hereby incorporated by reference in itsentirety. A commercially available line of Enhanced Fiber Additive isavailable from Cargill, Incorporated, Minneapolis, Minn., under thetrade name HemiForce™. For purposes of this invention, the amount ofEnhanced Fiber Additive in the soft tissue can be from about 0.5 toabout 5 dry weight percent, more specifically from about 0.5 to about 4dry weight percent, and still more specifically from about 1 to about 3dry weight percent.

The bone dry basis weight of the soft tissues of this invention can befrom about 15 to about 15 to about 35 grams per square meter (gsm), morespecifically from about 15 to about 30 gsm, and more specifically fromabout 15 to about 25 gsm.

The geometric mean tensile strength (GMT) of the soft tissues of thisinvention can be from about 500 to about 1000 grams per 3 inches ofwidth, more specifically from about 500 to about 900 grams per 3 inchesof width, and still more specifically from about 550 to about 650 gramsper 3 inches of width. For purposes of simplicity, the GMT is sometimesreported as “grams”.

The Dispersibility of the soft tissues of this invention can be about1.5 cycles or less, more specifically from about 0.5 to about 1.5cycles, more specifically from about 0.5 to about 1.0 cycle, and stillmore specifically about 1.0 cycle.

The opacity of the soft tissues of this invention can be from about 42.0to about 47.0 percent, more specifically from about 42.0 to about 46.5percent, and still more specifically from about 42.5 to about 46.5percent.

The fiber opacity efficiency for the tissues of this invention, which isthe ratio of the opacity divided by the bone dry basis weight and is ameasure of the efficiency of the fibers in providing opacity to thetissue sheet, can be about 2.5 percent/gsm or greater, more specificallyfrom about 2.5 to about 3.0 percent/gsm , and still more specificallyfrom 2.56 to 2.70 percent/gsm.

Suitable papermaking fibers particularly include, without limitation,softwood and hardwood fibers. As used herein, the term “furnish” meansthe papermaking fibers, such as the softwood and hardwood fibers, usedto make the tissue, excluding other furnish components or additives,such as EFA. The amount of softwood fibers in the furnish can be fromabout 5 to about 40 dry weight percent, more specifically from about 10to about 40 percent, more specifically from about 10 to about 30percent, and still more specifically from about 10 to about 20 percent.Similarly, the amount of hardwood fibers in the furnish can be fromabout 60 to about 95 dry weight percent, more specifically from about 60to about 90 dry weight percent, more specifically from about 70 to about90 percent, and still more specifically from about 80 to about 90 dryweight percent. Relatively speaking, higher amounts of softwood fiberswill increase tensile strength, while higher levels of hardwood fiberswill increase surface softness and opacity.

In the interests of brevity and conciseness, any ranges of values setforth in this specification contemplate all values within the range andare to be construed as written description support for claims recitingany sub-ranges having endpoints which are whole number or otherwise oflike numerical values within the specified range in question. By way ofa hypothetical illustrative example, a disclosure in this specificationof a range of from 1 to 5 shall be considered to support claims to anyof the following ranges: 1-5; 1-4; 1-3; 1-2; 2-5; 2-4; 2-3; 3-5; 3-4;and 4-5. Similarly, a disclosure in this specification of a range from0.1 to 0.5 shall be considered to support claims to any of the followingranges: 0.1-0.5; 0.1-0.4; 0.1-0.3; 0.1-0.2; 0.2-0.5; 0.2-0.4; 0.2-0.3;0.3-0.5; 0.3-0.4; and 0.4-0.5. In addition, any values prefaced by theword “about” are to be construed as written description support for thevalue itself. By way of example, a range of “from about 1 to about 5” isto be interpreted as also disclosing and providing support for a rangeof “from 1 to 5”, “from 1 to about 5” and “from about 1 to 5”.

Test Methods

As used herein, sheet “bulk” is calculated as the quotient of the sheet“caliper” (hereinafter defined), expressed in microns, divided by thebasis weight, expressed in grams per square meter. The resulting sheetbulk is expressed in cubic centimeters per gram. More specifically, thesheet caliper is the representative thickness of a single sheet measuredin accordance with TAPPI test methods T402 “Standard Conditioning andTesting Atmosphere For Paper, Board, Pulp Handsheets and RelatedProducts” and T411 om-89 “Thickness (caliper) of Paper, Paperboard, andCombined Board” with Note 3 for stacked sheets. The micrometer used forcarrying out T411 om-89 is an Emveco 200-A Tissue Caliper Testeravailable from Emveco, Inc., Newberg, Oreg. The micrometer has a load of2 kilo-Pascals, a pressure foot area of 2500 square millimeters, apressure foot diameter of 56.42 millimeters, a dwell time of 3 secondsand a lowering rate of 0.8 millimeters per second.

As used herein, the “geometric mean tensile strength” is the square rootof the product of the machine direction tensile strength multiplied bythe cross-machine direction tensile strength. The “machine direction(MD) tensile strength” is the peak load (grams-force) per 3 inches (76.2mm) of sample width when a sample is pulled to rupture in the machinedirection. Similarly, the “cross-machine direction (CD) tensilestrength” is the peak load per 3 inches (76.2 mm) of sample width when asample is pulled to rupture in the cross-machine direction. The“stretch” is the percent elongation of the sample at the point ofrupture during tensile testing. The procedure for measuring tensilestrength is as follows.

Samples for tensile strength testing are prepared by cutting a 3 inches(76.2 mm) wide by 5 inches (127 mm) long strip in either the machinedirection (MD) or cross-machine direction (CD) orientation using a JDCPrecision Sample Cutter (Thwing-Albert Instrument Company, Philadelphia,Pa., Model No. JDC 3-10, Serial No. 37333). The instrument used formeasuring tensile strengths is an MTS Systems Sintech 11S, Serial No.6233. The data acquisition software is MTS TestWorks® for Windows Ver.3.10 (MTS Systems Corp., Research Triangle Park, N.C.). The load cell isselected from either a 50 Newton or 100 Newton maximum, depending on thestrength of the sample being tested, such that the majority of peak loadvalues fall between 10-90% of the load cell's full scale value. Thegauge length between jaws is 4±0.04 inches (101.6±1 mm). The jaws areoperated using pneumatic-action and are rubber coated. The minimum gripface width is 3 inches (76.2 mm), and the approximate height of a jaw is0.5 inches (12.7 mm). The crosshead speed is 10±0.4 inches/min (254±1mm/min), and the break sensitivity is set at 65%. The sample is placedin the jaws of the instrument, centered both vertically andhorizontally. The test is then started and ends when the specimenbreaks. The peak load is recorded as either the “MD tensile strength” orthe “CD tensile strength” of the specimen depending on direction of thesample being tested. At least six (6) representative specimens aretested for each product or sheet, taken “as is”, and the arithmeticaverage of all individual specimen tests is either the MD or CD tensilestrength for the product or sheet.

The “geometric mean slope” (GM Slope) is the square root of the productof the machine direction tensile slope and the cross-machine directiontensile slope. The tensile slope is the least squares regression slopeof the load/elongation curve described above measured over the range of70-157 grams (force). The slope is in kilograms per unit elongation(i.e. 100% strain) for a 76.2 mm (3 inches) wide sample, but forpurposes of simplicity sometimes reported herein as “kilograms”.

As used herein, “Dispersibility” is a measure of the propensity of atissue product to break apart when placed in water under mild agitation.It is determined by placing a sample of the product into a slosh box andobserving the dynamic break-up of the sample as the slosh box tips(cycles) back and forth. For rolls of bath tissue, the sample to betested is a single “sheet” which, for purposes herein and wellunderstood within the tissue industry, is the segment of the bath tissuesheet located between consecutive lines of perforation. It can consistof one or more plies. Such sheets are typically about 4 inches square.The actual size, however, is not particularly important since the sizeof the slosh box is sufficiently large to accommodate any known tissuesheets. For purposes of testing tissue sample basesheets, which have notbeen converted into actual final product, a 4 inches-by-4 inches sampleis sufficient.

The slosh box used for the dynamic break-up of the sample consists of aplastic box having inside dimensions measuring 18 inches wide (as viewedfrom the front), 12 inches deep (front to back) and 6.5 inches high. Itis constructed from 0.5 inch thick Plexiglas® and is provided with atightly fitting lid. The slosh box rests securely on a rocking platformand rocks back and forth from short side (12 inch end) to short side(opposite 12 inch end). The underside of the platform is attached to areciprocating cam. In operation, the rotational movement of the camcyclically raises one side of the platform and thereby also lowers thecorresponding side of the slosh box, pivoting at the center of the box.The amplitude of the rocking motion of the one side of the slosh box is±2 inches (a range of 4 inches from the top to the bottom of the rockingcycle). The rotational speed of the cam is set to a constant speed of 26revolutions per minute (±2 revolutions per minute), which results in 43slosh cycles per minute. For purposes herein, a “cycle” consists of one“up and down” motion of the slosh box.

Prior to testing, the slosh box is filled with 2000 ml±20 ml of a soaksolution. The soak solution consists of distilled water mixed with 0.25teaspoon of sodium bicarbonate in order to keep the pH of the soaksolution higher than 7. The temperature of the soak solution ismaintained at 23° C.±3° C. Solution is drained and the box chamber isrinsed and refilled between each specimen characterization. To carry outthe test, the tissue sample is placed flat on the surface of the waterin the slosh box and the slosh box is started immediately. The break-upof the sample in the slosh box is visually observed and the number ofcomplete cycles required to separate the sample into two distinct piecesis recorded. (For multi-ply products, ply separation does not constituteseparation of the sample into two distinct pieces for purposes of thistest. Instead, at least one of the plies must separate into two distinctpieces.) Five replicates of the tissue sample are tested. The observednumber of cycles needed to break up the test samples is averaged toachieve a Dispersibility value (in “cycles”) for the product sample.

As used herein, “opacity” is measured using a Technibrite Micro TB-1Ctester, which is well known in the paper industry, available fromTechnidyne Corporation, 100 Quality Avenue, New Albany, Ind., USA. TheTechnibrite Micro TB-1C tester, which is a dual beam optical system, isa fully automatic microprocessor-controlled instrument that providesbrightness, color, opacity and fluorescence in conformance with ISO andother international standards. Tests are conducted in a standardlaboratory atmosphere (23° C.±1° C. and 50%±2% humidity) following theinstructions for the instrument. For measuring tissue opacity, the QCroutine is used with the black body cup and with the Y (green) filter inthe active position. When taking measurements, the operator should avoidtaking readings in areas of the sample which contain printing orperforations. Measurements should be taken on the outside of the sheet(the side of the sheet that consumers would see). Fifteen representativesamples should be tested and the results averaged to obtain a value forthe particular product. The measurement values represent reflectance andare expressed as a percent.

EXAMPLES

In order to further illustrate this invention, a number of tissues wereproduced using conventional creped, wet-pressed technology, such as themethod disclosed in U.S. Pat. No. 6,368,454 entitled “Method of MakingSoft Bulky Single Ply Tissue” issued Apr. 9, 2002, to Dwiggins et al.(without embossing), which is hereby incorporated by reference. Unlessstated otherwise, the particular tissue making method used is notcritical.

Example 1 (Invention)

Single-ply bath tissue basesheet was produced in a conventional manneron a pilot scale tissue machine. More particularly, a tissue web wasformed on a forming fabric, transferred to a felt, and thereaftertransferred to a Yankee dryer in a conventional manner. The tissue webwas dried to approximately 95 percent consistency on the Yankee dryerand creped using standard creping technology. The resulting crepedtissue sheet was wound into a parent roll for testing.

The tissue furnish was a blended furnish comprising eucalyptus hardwood(HW) fibers and refined northern softwood kraft (SW) fibers. Prior toformation of the web, the northern softwood fibers were pulped for 30minutes at 2.5 percent consistency, while the eucalyptus hardwood fiberswere pulped at 2 percent consistency. The northern softwood fibers wererefined for 5 minutes. The pulp mix (expressed as bone dry weightpercent) was 39.2 percent SW, 58.8 percent HW, and 2 percent CargillHemiForce™ Enhanced Fiber Additive (EFA). The EFA was diluted to below 2percent consistency and allowed to mix in the blended stock chest for 20minutes before starting formation of the tissue web. The tissue machinespeed (the speed of the Yankee dryer) was 50 feet per minute (fpm).

Example 2 (Invention)

A single-ply bath tissue was made as described in Example 1, except thepulp mix was 19.8 percent SW, 79.2 percent HW and 1 percent EFA.

Example 3 (Invention)

A single-ply bath tissue was made as described in Example 1, except thepulp mix was 9.7 percent SW, 87.3 percent HW and 3 percent EFA.

Example 4 (Control 1)

A single-ply bath tissue was made as described in Example 1, except thesoftwood fibers were not refined and the pulp mix was 40 percent SW and60 percent HW. No EFA was added to the pulp mix.

Example 5 (Control 2)

A single-ply bath tissue was made as described in Example 1, except thesoftwood fibers were refined for 9 minutes and the pulp mix was 40percent SW and 60 percent HW. No EFA was added to the pulp mix.

Example 6 (Control 3)

A single-ply bath tissue was made as described in Example 1, except thepulp mix was 40 percent SW and 60 percent HW. No EFA was added to thepulp mix.

Example 7 (Control 4)

A single-ply bath tissue was made as described in Example 1, except thepulp mix was 39.6 percent SW, 59.4 percent HW and 1 percent EFA.

Examples 8-16 (Commercial)

A number of commercially-available bath tissue samples were collectedand tested for various properties. The furnish compositions are notknown.

All of the tissues were measured for bone dry basis weight, geometricmean tensile strength, Dispersibility (slosh box cycles) and opacity.The Invention 1 and 2 samples and the Control 3 and 4 samples were alsoranked for panel softness. These samples were chosen for softnesstesting since they all had approximately the. same geometric meantensile strength. The tissue samples were given to a trained panel whichranked the tissue samples for surface softness on a relative scale. Aranking of “A” is considered relatively softer than a ranking of “B”.The results are presented below in Table 1.

TABLE 1 Basis EFA Surface Weight Furnish (weight GMT DispersibilityOpacity Opacity/Basis Softness Example (gsm) (% SW/% HW) percent)(grams) (Cycles) (Percent) Weight Ratio (Grouping)  1-Invention 1 15.6939.2/58.8 2 651 1 42.332 2.70 B  2-Invention 2 18.06 19.8/79.2 1 554 146.276 2.56 A  3-Invention 3 17.00  9.7/87.3 3 637 1 46.008 2.71 4-Control 1 18.17 40/60 0 311 1 45.44 2.50  5-Control 2 18.54 40/60 0894 6.4 46.108 2.52  6-Control 3 18.73 40/60 0 586 2.6 46.158 2.46 B 7-Control 4 17.10 39.6/59.4 1 597 1.8 43.478 2.54 B  8-Marcal 16.30 —942 5 46.76 2.87  9-Kroger 15.57 — 601 5 40.24 2.58 10-Scott 1000 17.22— 773 6 44.63 2.59 11-Albertson's 17.68 — 684 2 44.29 2.51 12-Pert 16.39— 419 3 44.04 2.69 13-Walgreen's 19.16 — 800 4 52.95 2.76 14-CVS 16.39 —419 3 44.04 2.69 16-Homelife 17.43 — 895 7 41.31 2.37

The results show, with regard to softness, that the Invention 2 samplehad the highest surface softness (95% confidence level) among thetissues tested. The other inventive sample tested (Invention 1) wasjudged to have surface softness similar to that of the controls. Thisdata indicates that the inventive tissues were at least as soft as thecontrol codes.

The results further show that the tissues of this invention have animproved Dispersibility and better opacity (as measured by the ratio ofthe opacity divided by the basis weight) at a fixed geometric meantensile strength. Obtaining the desired combination of softness, tensilestrength, Dispersibility and fiber opacity efficiency required furnishand chemistry manipulation. As the data of Table 1 indicates, theControl 1 sample had very good Dispersibility (1 cycle) by not refiningthe softwood fibers in the furnish. However, that sample also had a verylow geometric mean tensile strength of 311 grams. On the other hand, theControl 4 sample, which included 1 dry weight percent EFA in the fiberfurnish, had a good geometric mean tensile strength of 597 grams, but ahigher Dispersibility (1.8 cycles), which was only somewhat better thanthe best of the commercially available bath tissues (2 cycles for theAlbertson's product). Therefore, to further improve the Dispersibility,the furnish was adjusted for the inventive samples to include additionalhardwood fiber and/or EFA. For the Invention 1 sample, the EFA contentof the furnish was increased to 2 dry weight percent. For the Invention2 and 3 samples, the hardwood portion of the furnish was increased toapproximately 80 percent and 90 percent, respectively. In all cases, theDispersibility was 1 cycle and the geometric mean tensile strength wasabove 550 grams.

Without being bound by theory, it is believed the increase in thehardwood portion of the furnish increased the Dispersibility of theproduct (i.e. reduced the number of cycles) and increased the opacity ofthe product. This is thought to be due to the higher fiber count andreduced fiber length associated with the substitution of hardwood fiberfor a portion of the softwood fiber. The higher fiber count of thehardwood pulp is thought to have increased the opacity and the lowerfiber length is thought to have increased the product Dispersibility.However, the increase in hardwood fiber content also decreased thetensile strength of the product, perhaps reducing it below the requiredstrength level. This was countered, where necessary, by furtherincreasing the level of EFA as the EFA provided improved strength andfiber opacity efficiency as well as good Dispersibility.

It will be appreciated that the foregoing examples, given for purposesof illustration, are not to be construed as limiting the scope of thisinvention, which is defined by the following claims and all equivalentsthereto.

1. A soft tissue having a bone dry basis weight from about 15 to about35 grams per square meter, from about 5 to about 40 dry weight percentsoftwood fibers and from about 60 to about 95 dry weight percenthardwood fibers, a geometric mean tensile strength from about 500 toabout 1000 grams per 3 inches of width and a Dispersibility of about 1.5cycles or less.
 2. The tissue of claim 1 having a Dispersibility fromabout 0.5 to about 1.5 cycles.
 3. The tissue of claim 1 having aDispersibility from about 0.5 to 1.0 cycles.
 4. The tissue of claim 1having a Dispersibility of about 1 cycle.
 5. The tissue of claim 1having a ratio of opacity divided by bone dry basis weight of about 2.5percent/gsm or greater.
 6. The tissue of claim 1 having a ratio ofopacity divided by bone dry basis weight from about 2.5 to about 3.0percent/gsm.
 7. The tissue of claim 1 having a ratio of opacity dividedby bone dry basis weight from 2.56 to 2.70 percent/gsm.
 8. The tissue ofclaim 1 consisting of a single ply.
 9. The tissue of claim 1 having fromabout 10 to about 20 dry weight percent softwood fibers and from about80 to about 90 dry weight percent hardwood fibers.
 10. A soft tissuehaving a bone dry basis weight from about 15 to about 35 grams persquare meter, from about 5 to about 40 dry weight percent softwoodfibers and from about 60 to about 95 dry weight percent hardwood fibers,and from about 0.5 to about 5 dry weight percent of an Enhanced FiberAdditive, said tissue having a geometric mean tensile strength fromabout 500 to about 1000 grams per 3 inches of width and a Dispersibilityof about 1.5 cycles or less.
 11. The tissue of claim 10 having fromabout 0.5 to about 4 dry weight percent of an Enhanced Fiber Additive.12. The tissue of claim 10 having from about 1 to about 3 dry weightpercent of an Enhanced Fiber Additive.
 13. The tissue of claim 10 havinga Dispersibility from about 0.5 to 1.5 cycles.
 14. The tissue of claim10 having a Dispersibility of about 1 cycle.
 15. The tissue of claim 10having a ratio of opacity divided by bone dry basis weight of about 2.5percent/gsm or greater.
 16. The tissue of claim 10 having a ratio ofopacity divided by bone dry basis weight from about 2.5 to about 3.0percent/gsm.
 17. The tissue of claim 10 having a ratio of opacitydivided by bone dry basis weight from 2.56 to 2.70 percent/gsm.
 18. Thetissue of claim 10 consisting of a single ply.
 19. The tissue of claim10 having from about 10 to about 20 dry weight percent softwood fibersand from about 80 to about 90 dry weight percent hardwood fibers.